1. Field of the Invention
The present invention relates to an image forming apparatus for image formation on a sheet, such as a copier.
2. Description of the Prior Art
Conventional electrostatic recording apparatus such as electrophotographic copiers have been associated with the drawbacks that the charging characteristics of the recording member, i.e. photosensitive drum, are affected by environmental parameters such as moisture and also by time-dependent changes such as mechanical fatigue during use.
In order to compensate such environmental or time-dependent effects, there has been proposed a method of controlling the image recording conditions such as the conditions of charging, exposure, developing etc. to attain optimum formation in response to the detected potential of the latent image formed on the photosensitive drum, or a method of controlling the conditions of charging and exposure in response to the charging period to attain optimum image recording.
Such conventional methods are however defective in that the apparatus becomes inevitably expensive since the detector for the surface potential is complicated and expensive.
Also such conventional methods are unable to compensate for a rapid fluctuation in the charging characteristic of the photosensitive drum immediately after the start of electrostatic charging. Such fluctuation will hereinafter be referred to as initial fluctuation.
Said initial fluctuation of the photosensitive drum, a phenomenon that the charging characteristic shows a rapid fluctuation during a short period immediately after the start of electrostatic charging due to the past charging hysteresis before it reaches a stable state, has become a problem in recent years since the idle waiting time in the copiers has become limited due to the increasing copying speed. As shown by a curve in FIG. 1, the dark potential V.sub.D, which is a drum surface potential formed by an exposure corresponding to a dark image area, shows an exponential change from an initial value Vt.sub.0 to a final value Vt.sub..infin. according to a time constant .tau..sub.1 when the charging is initiated at t.sub.0 after a sufficiently long pause. Said potential change may appear in inverted form as represented by a curve B depending on the charging hysteresis, i.e. the charging state when the preceding charging is terminated.
Also if the charging is initiated for example at t.sub.2 after an insufficiently long pause, the surface potential shows a change with the same time constant .tau..sub.1 from a different initial value (Vt.sub.2) to the afore-mentioned final value Vt.sub..infin., as shown by a curve A' or B'. Consequently the charge starting voltage varies from Vt.sub..infin. to Vt.sub.0 with a time constant .tau..sub.2 (&gt;.tau..sub.1) as shown by curves C and D in FIG. 1. In such situation the potential difference .DELTA.E=Vt.sub..infin. -Vt.sub.0 and the time constants .tau..sub.1, .tau..sub.2 vary according to the charging hysteresis and environmental parameters, particularly humidity, and show fluctuations among different production lots of the photosensitive drum. Such initial fluctuation of the potential causes a density difference between the first and second copies or within a copy.
Also such conventional control methods are not effective enough since a predetermined control factor is always used even for the deterioration of the photosensitive drum or a slow change in the environmental parameters, and are therefore unable to provide optimum image recording conditions. Furthermore, in such methods it is difficult to realize the desired recording conditions through frequently repeated control steps since the control procedure is not real-time and since the lamp utilized for forming light and dark image areas requires relatively long starting and extinguishing periods. Furthermore the eventual fluctuation of the characteristic between different production lots of the photosensitive drum necessitates lengthy adjustment procedure at each drum replacement in order to compensate the difference between the drum characteristic and the control factor, and such difference may retard the establishment of the optimum conditions or cause an excessive compensation giving rise to an unstable control.